2-Stroke Engine

2-Stroke Engine
(Source-roadandtrack)

2-Stroke engine is typically an IC engines that takes 2 full strokes for completing a cycle. Each stroke rotates the crankshaft by 180 degrees, hence one complete rotation is achieved by the end of the 2 strokes. German inventor Karl Benz in 1880 invented it.

Meaning of 2-stroke

2-stroke actually means two strokes of the piston. We can say that the movement of the piston from the TDC to BDC makes one stroke and the movement of the piston from BDC to TDC makes another stroke. So, combining the two processes, it makes two strokes of the piston, i.e., the movement of the piston from TDC and back to TDC. During this journey of the piston, there occurs two processes once, i.e., compression and exhaustion. One complete revolution of the crankshaft gives one power stroke. As the name implies, the entire cycle, i.e., suction, compression, power and exhaust happen in entire 2-stroke of the engine piston.

Valve timings

The actual timing of suction, compression, power and exhaust varies from engine-to-engine with respect to design and construction features such as:

  • Stroke bore ratio.
  • Engine rating.
  • Engine rpm
  • Connecting rod–crank length ratio.
Valve timing diagram of 2-stroke engine
Valve timing diagram of 2-stroke engine
Inlet(scavenge) opens42 deg. before BDC
Inlet closes42 deg. after BDC
Exhaust opens75 deg. before BDC
Exhaust closes60 deg. after BDC
Injection starts16 deg. before TDC
Injection ends20 deg. after TDC

Parts of 2-stroke engine

A two-stroke engine is comprised of the following parts:

  • Cylinder
  • Cylinder head
  • Piston
  • Piston rings
  • Connecting rod
  • Crank
  • Crankcase
  • Crankshaft
  • Ports
  • Spark plug(For Petrol engine)
  • Fuel Injector(For Diesel engine)
2-Stroke Marine Engine
2-Stroke Marine Engine

Analysis of the 2-Stroke

Up-stroke (Compression stroke):

The scavenge port is uncovered, exhaust valve is closed and pressurized air is fed into the cylinder. The introduction of the fresh air into the cylinder does the scavenging process, i.e.it removes all the exhaust gas from the previous cycle. This introduction of the scavenge air is done by the turbocharger. boosting of the air via air cooler by increasing the density of the air and improving the thermal efficiency. So basically, the turbocharger plays a major role in increasing the scavenge air to increase the thermal efficiency.
The piston then moves upwards (BDC to TDC), the exhaust valve closes, the scavenge ports are covered and the piston starts compressing the air. At the end of the stroke, BDC to TDC, the air pressure in the cylinder builds up to 32-45 Bars and there will be a corresponding rise in the temperature as well,i.e.,650-800°C.

Compression stroke
Compression stroke

Down-stroke (Power stroke):

The fuel is then injected into the cylinder by the fuel inductor to the hot compressed air. Then the fuel starts upon reaching the self ignition temperature of the fuel. The combustion causes the exhaustion of the gas which pushes the piston downwards, i.e., towards BDC. The piston being pushed down towards BDC by the combustion gases is doing some work. Hence, it is known as power/expansion stroke. The exhaust valve is then opened 40-70 deg. of the crankshaft rotation just before BDC. The exhaust gas rushes towards the atmosphere which lowest the cylinder pressure to 2-4 bars approximately. The temperature is high due to the exhaust gases, i.e.,250-500°C.Exhaust valves are open for about 118 to 130 degree of the crankshaft rotation. The scavenge ports are open for about 100 to 140 degree of the crankshaft rotation.

Power stroke
Power stroke

Why 2-stroke Engine is better?

  • 2-stroke engines are lighter, less bulky and economical for manufacturing.
  • Fuel selection: It uses look cost fuel as it is able to burn low grade fuels.
  • Efficiency: Both the thermal and engine efficiency of the two-stroke engine is more.
  • Power: It produces more power due to more power to weight ratio.
  • Reliability: More reliable and offers less maintenance as fewer moving parts are involved.
  • Direction control: Easy to reverse the direction as a smaller number of valves make it less complicated.
  • No reduction gear is required in the two stroke engine as they are slow speed engine.

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